The Effects of Physical Stressors on Bacterial Inactivation Rates in Biosolids

by O'Shaughnessy, Susan Ann.

Abstract (Summary)

Sanitation is fundamental to reducing disease and sustaining a high standard of
living. The evolution of sewer systems and the modern engineering of wastewater
treatment plants work to decrease health risk and manage environmental concerns
associated with the reuse and disposal of treated effluent and solid wastes generated as
byproducts. The recycling of treated solid wastes (biosolids) continues to be an
environmental challenge due to the shear volume produced, and its potentially hazardous
composition.
Solar drying of biosolids was studied in semi-arid regions as a sustainable method
for reducing pathogens. The initial studies were performed with no intervening
treatments. Average fecal coliform inactivation rates for digested biosolids during
summer experiments were determined to be 0.17 ± 0.03/day-1 and 0.17 ± 0.04/day-1,
respectively. Salmonella inactivation rates in aerobically digested biosolids were 0.11 ±
0.08 day -1 and 2.0 ± 2.0 day-1 for aerobically and anaerobically digested biosolids,
respectively for the summer seasons. Solar drying during warm dry seasons was
effective in reducing pathogens.
Microbial testing to verify the quality of biosolids can be expensive. Utilizing a
mathematical model to predict pathogen density levels during the solar drying process
can minimize such testing. The first order mathematical model, N
t
= N 10
-kdt
o
* where
the inactivation constant, kd, is further defined as a function of moisture (?) and
k
1 T
k
d
* * k
3
temperature (T), i.e. kd = f(?,T): k
1
k ÷
2
æ ö æ ö
= ç ÷ ç
è + Q ø è - T ø , k1 = 0.112, k2 = -
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41.88, and k3 = -0.5357; for all T greater than or equal to 38ºC, T=38ºC provided a good
estimate of the inactivation rate of fecal coliforms in biosolids. During subsequent field
studies, treatments were employed to manage the drying cycle of biosolids—tilling
increased the rate of drying, a covered solar drying bed increased the inactivation rate of
fecal coliforms by 300%, and an automated rain shield was engineered to limit enteric
bacterial regrowth due to rainfall.
Finally, since biosolids are to be considered a source of nitrogen when land-applied,
temporal samples of biosolids from various solar drying experiments were analyzed to
ascertain the levels of NH+4-N and NO-3-N throughout the drying process. Chemical
analyses revealed that as much as 34-92% of nitrogen was lost via volatilization during
the drying process.
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